1. Field of the Invention
The present invention relates to a method of driving an EL display device having a panel for performing gradation display in which a plurality of pixels are arranged in a matrix shape and the length during which each pixel is turned on is controlled. In particular, the present invention relates to a method of driving an EL display device having an EL panel using organic EL elements.
2. Description of the Related Art
The introduction of digital technology to equipment and systems used by broadcasters is increasing, and in recent years, the digitalization of broadcast radio waves, that is, research and development toward the realization of digital broadcasting, is being performed worldwide.
Further, dealing with the digitalization of broadcast radio waves, research and development has also been thriving in recent years for active matrix display devices in which a digital video signal having image information is used as is without being converted to analog, to display an image.
A surface area division driving method and a time division driving method can be given as methods for performing gradation display in accordance with the two voltage values of a digital video signal.
The surface area division driving method is a driving method for performing gradation display by dividing one pixel into a plurality of subpixels, and driving each subpixel independently based upon a digital video signal. One pixel must be divided into a plurality of subpixels in this surface area division driving method. In addition, it is also necessary to form pixel electrodes corresponding to the respective subpixels in order to drive the divided subpixels independently. A difficulty thus develops in that the pixel structure is complex.
On the other hand, the time division driving method is a driving method for performing gradation display by controlling the length of time during which pixels are turned on. Specifically, one frame period is divided into a plurality of display periods. Each pixel is then placed in a turned on or turned off state in each display period in accordance with a digital video signal. The gradation of a certain pixel is found by summing the length of the display periods which that pixel is turned on within all of the display periods appearing within one frame period.
In general, the response speed of organic EL materials is fast compared to liquid crystals and the like, and therefore the organic EL materials are suitable for time division drive.
Binary code is convenient in realizing high-level gradations when performing time division drive. A case of displaying mid-level gradations by time division drive in accordance with a simple binary code method is explained in detail below using FIGS. 19A and 19B.
FIG. 19A shows a pixel portion of a display device, and the lengths of all display periods appearing within one frame period in the pixel portion are shown in FIG. 19B.
An image is displayed using a 6 bit digital video signal capable of displaying gradations 1 to 64 in FIGS. 19A and 19B. The right half of the pixel portion performs display of 33 (32+1) gradations, and the left half of the pixel portion performs display of 32 (31+1) gradations.
Six display periods (display periods Tr1 to Tr6) generally appear within one frame period when using a 6 bit digital video signal. The first to the sixth bits of the digital video signal correspond to the display periods Tr1 to Tr6 respectively.
The ratio of lengths of the display periods Tr1 to Tr6 becomes 20:21:22:23: 24:25. The length of the display period Tr6 corresponding to the most significant bit (the sixth bit in this case) of the digital video signal is the longest, and the length of the display period corresponding to the least significant bit (the first bit) of the digital video signal is the shortest.
For a case of performing display of the 32nd gradation, the pixels are placed in an on state in the display periods Tr1 to Tr5, and the pixels are placed in an off state during the display period Tr6. Further, for a case of performing display of the 33rd gradation, the pixels are placed in a turned off state during the display periods Tr1 to Tr5, and are turned on during the display period Tr6.
A pseudo contour may be visible at a boundary portion between the portion for performing display of the 32nd gradation and the portion for performing display of the 33rd gradation.
The term pseudo contour refers to an unnatural contour line repeatedly visible when performing time gradation display in accordance with a binary code method, and it is said that the main cause of the pseudo contour is fluctuations which develop in the perceived brightness due to the characteristics of human vision. A mechanism of the generation of the pseudo contour is explained using FIGS. 20A and 20B.
FIG. 20A shows a pixel portion of a display device in which a pseudo contour develops, and FIG. 20B shows the ratio of the lengths of display periods appearing within one frame period in the pixel portion.
An image is displayed using a 6 bit digital video signal capable of displaying gradations 1 to 64 in FIGS. 20A and 20B. The right half of the pixel portion performs display of 33 gradations, and the left half of the pixel portion performs display of 32 gradations.
The pixels are placed in an on state during 31/63 of one frame period, and the pixels are placed in an off state during 32/63 of the one frame period, in portions of the pixel portion performing display of the 32nd gradation. Periods during which the pixels are turned on appear alternately with periods in which the pixels are turned off.
Further, the pixels are placed in an on state during 32/63 of one frame period and the pixels are placed in an off state during 31/63 of the one frame period, in portions of the pixel portion performing display of the 33rd gradation. Periods during which the pixels are turned on appear alternately with periods in which the pixels are turned off.
For a case of displaying a moving picture, the interface between portions displaying the 32nd gradation and portions displaying the 33rd gradation in FIG. 20A is taken, for example, as moving in the direction of the dotted line. Namely, the pixels near the boundary switch over between displaying the 32nd Gradation and displaying the 33rd gradation. By doing so, a turn on period for displaying the 33rd gradation begins immediately after a turn on period for displaying the 32nd gradation in the pixels near the boundary. The human eye thus can see the pixels turned on continuously during one frame period. This is perceived as an unnatural bright line on the screen.
Conversely the interface between the portions displaying the 32nd gradation and the portions displaying the 33rd gradation in FIG. 20A is taken, for example, as moving in the direction of the solid line. Namely, the pixels near the boundary switch over between displaying the 33rd gradation and displaying the 32nd gradation, By doing so, the turn on period for displaying the 32nd gradation begins immediately after the turn on period for displaying the 33rd gradation in the pixels near the boundary. The human eye thus can see the pixels turned off continuously during one frame period. This is perceived as an unnatural dark line on the screen.
These types of unnatural bright lines and dark lines appearing on the screen are obstructions to display, referred to as pseudo contours (moving pseudo contours).
Obstructions to display also become visible in static images due to the same cause as that in which the moving pseudo contours develop in moving images. The obstructions to display static images are one in which flickering motion can be seen in the boundaries of gradations. A simple explanation of the reason that this type of obstructions to display is visible in static images is made below.
Even if a person's eye is fixed upon one point, the visual point moves slightly, and it is difficult to stare at one point with certainty. Therefore, even if ones intention is to stare at the boundary between portions of the pixel portion in which the pixels are performing display of the 32nd gradation, and portions in which the pixels are performing display of the 33rd gradation, the visual point will actually move slightly left and right, up and down.
For example assume that the visual point moves from portions performing display of the 32nd gradation to portions performing display of the 33rd gradation as shown by the dashed line. For a case in which the pixels are in a turned off state when the visual point is located in portions displaying the 32nd gradation, and the pixels are in a turned off state when the visual point is located in portions displaying the 33rd gradation, the pixels are seen to be in a turned off state through the entire one frame period by an observer's eves.
Conversely, for example, assume that the visual point moves from portions performing display of the 33rd gradation to portions performing display of the 32nd gradation, as shown by the solid line. For a case in which the pixels are in a turned on state when the visual point is located in portions displaying the 33rd gradation, and the pixels are in a turned on state when the visual point is located in portions displaying the 32nd gradation, the pixels are seen to be in a turned on state through the entire one frame period by an observer's eyes.
The pixels are therefore seen by human eyes to be in a turned on state, or in a turned off state, throughout one frame period because of the tiny movement to the left and right, up and down, of the visual point, and obstructions to display in which the boundary portion is seen to sway back and forth is seen.